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13 Historic Groundwater Condition% Assessment of petroleum hydrocarbon impact in groundwater has been performed at this site since July 1987 Historic groundwater monitoi ing indicates that decreases of petroleum hydrocarbon concentrations have partly occurred due to natural degradation Recent groundwater monitoring indicates that PSC's remedial efforts are having a substantial affect on further reducing petroleum hydrocarbon concentrations Benzene concentrations in groundwater samples collected from well MW-1, located near the source, have ranged from a high of 6,800 micrograms per later (µg/L) in July 1987 to non detect in November 2002 (see Table 4) Total petroleum hydrocarbons as gasoline (TPH-G) concentrations in samples from MW-1 have ranged from a high of 196,000 µg/L in April 1993 to a low of 110 µg/L measured in November 2002 Current petroleum hydrocarbon concentrations are further discussed in Section 3 2 of this report Oxygenate compounds methyl-tert-butyl ether (MTBE), ethyl-tert-butyl ether (ETBE), and tert-amyl-methyl ether (TAME) have historically not been detected in site groundwater samples (see Table 5) MTBE was detected in one groundwater sample collected in November 2002, however, its detection- is considered potentially anomalous The compound tert-butanol (TBA) was detected in one groundwater sample collected from MW-1 on November 8, t 1999, and again in PZ-1 samples collected on October 18, 2001 and April 20, 1� 2001, but has never been detected in any other groundwater sample collected at the site until this most recent Quarter The detection of TBA in two groundwater samples collected in November 2002 is considered potentially anomalous The oxygenate compound da-isopropyl ether (DIPS) has historically been measured at concentrations of 140 µg/L or less in samples collected from wells near the source of the release Current oxygenate / ' concentrations are further discussed in Section 3 4 of this report As shown in Table 1 and Table 2, rent depths to groundwater average about 40 feet below ground surface (bgs) The lowest depth to groundwater, observed during our historic site monitoring, was about 60 feet bgs in 1 ' December 1992 The EHD suggests that groundwater was at a depth of about 80 feet in 1978 Groundwater has consistently been evaluated to flow in an east-northeast direction at approximately N70E across the site Current groundwater elevation information is summarized in Section 3 1 of this report 14 Remedial Action On April 25, 2001, PSC initiated operation of an air spargang and soil venting remediation system to address subsurface petroleum hydrocarbon impact Air spargang was initially performed in well SP-1 However, PSC suspected that t 6 NmcntUm 2002 Groundwatr,r Monitoring And Rcnttdiation Status Report i i hr,LarYhgrams Baking Contpin} 20�] tiouth Airport Way-StOLLtOn Califon-' Prr,pamd for 1 Ylt I %Rtvc R VnS B ki,INC C�OMPANN Prepared by Philip S6rV1e65 Corp mitt NV�624001j2u21,921 c%S,wck n%�r IoLAronu 10-1inon JOS DLCLmbLf 9 2002 r